1. Field of the Invention
The present invention relates to an image forming apparatus such as copying apparatus, printer, or the like and, more particularly, to an image forming apparatus in which a heat-fixing unit has been provided.
2. Description of the Related Art
An image forming apparatus of an electrophotographic recording system forms a toner image onto a recording material, thereafter, executes a heat-fixing process to the toner image, and discharges the recording material onto a discharge tray. A schematic construction of a laser beam printer (LBP) as an example of such an image forming apparatus will now be described with reference to FIG. 3. The LBP illustrated in FIG. 3 is a printer for forming a monochromatic image.
In the diagram, a processing unit 101 including a photosensitive drum 102 is arranged in a main body of the image forming apparatus. A laser beam modulated according to image information is irradiated onto the photosensitive drum 102 from a laser scanner 106 and an electrostatic latent image is formed.
The processing unit 101 includes: a primary charger 103 for charging the photosensitive drum 102; a developing unit 104; a cleaning apparatus 105; and the like. Recording materials P on a feeding tray 114 are fed one by one and conveyed to the photosensitive drum 102 at predetermined timing by a registration roller 116.
The recording material P is pressed to the photosensitive drum 102 and the toner image is transferred onto the recording material P by a transfer roller 107. After that, the toner image is fixed by a heat-fixing portion 113 and the recording material P is discharged to the outside of the image forming apparatus through a discharge portion 120. In the example illustrated in the diagram, the recording material is discharged onto a discharge tray 118 provided integratedly with the apparatus main body.
Various devices for preventing such a situation that a discharge state of the recording material to be properly discharged onto the discharge tray 118 becomes incomplete are considered. One of them is a construction in which a projecting portion to kick out the recording material is formed on a peripheral surface of a roller of the discharge portion 120. The construction in which the projecting portion to kick out the recording material is formed on the peripheral surface of the roller of the discharge portion has been disclosed in each of Japanese Patent Application Laid-Open No. H05-039156, Japanese Patent No. 3187493, Japanese Patent Publication No. H07-017295, and Japanese Patent Application Laid-Open No. H05-221570.
An example of such a construction will now be described with reference to FIG. 4. The discharge portion 120 illustrated in FIG. 4 is constructed by: an discharge roller 120a which can rotate in the direction shown by an arrow; and a pressing roller 120b which is come into pressure contact with a peripheral surface of the discharge roller 120a by an urging unit (not illustrated). In a simplex print mode, the discharge roller 120a is located on an image surface side and the pressing roller 120b is located on a non-image surface side. The pressing roller 120b is driven synchronously with the driving of the discharge roller 120a or rotated in association with the discharge roller 120a. 
A plurality of projecting portions 120c are arranged in both edge portions or near the both edge portions of the pressing roller 120b at predetermined intervals. Although the projecting portions 120c are provided for the pressing roller 120b in this example, there is also a case where the projecting portions are provided for the discharge roller 120a. In both of the above cases, the projecting portions 120c press a rear edge of the conveying direction of the recording material P in the discharge direction, thereby improving discharge performance of the recording material. In each of the discharge roller 120a and the pressing roller 120b illustrated in FIG. 4, a roller portion which comes into contact with the recording material is divided along the axial direction (such a roller is referred to as a divided roller). In the case of such a construction, since an area where the roller comes into contact in the image area on the recording material and an area where the roller is not come into contact occur, there is a case where a variation in glossiness of an image occurs (such a portion is called a roller mark). To suppress such a roller mark phenomenon, there is also a case of using a roller having a width wider than that of the image area of the recording material (such a roller is referred to as a non-divided roller). As illustrated in FIG. 5, a construction in which projecting portions 120e to kick out the recording material are provided in both edge portions of the non-divided roller is also considered. In the case of such a construction, since the projecting portions 120e do not rub the image on the recording material, the discharge performance can be improved while suppressing the occurrence of a defective image. Particularly, if a width of discharge roller 120f is set to be wider than that of the image area of the recording material P in a manner similar to a pressing roller 120d, the occurrence of the roller mark can be also suppressed.
If the non-divided roller having the projecting portions only in both edge portions in the axial direction is used as a roller of a discharge portion as mentioned above, the discharge performance can be improved while suppressing the occurrence of the defective image.
However, in the case of printing onto a recording material of a small size by a printer in which the non-divided roller having the projecting portions only in both edge portions in the axial direction is used as a roller of the discharge portion, since the recording material cannot be pushed out by the projecting portions, there is a possibility of deterioration of the discharge performance of the recording material of the small size. Particularly, in the case of printing onto the recording material of the small size, since control for decreasing a fixing speed is made to suppress over-rising of a temperature in an area of the fixing portion where the recording material does not pass, such a situation that the proper discharge performance cannot be assured is considered.